mongodb - 如何计算mongodb中嵌套文档的出现次数？

Question

在数组嵌套数组的情况下，如何计算特定值的个数？例如，我想统计下面文档中“答案”的数量。查询应该返回有 2 个苹果和 1 个香蕉。

{
  "_id" : ObjectId("52c1d909fc7fc68ddd999a73"),
  "name" : "Some survey",
  "questions" : [
    {
      "_id" : ObjectId("52c1e250fc7fc68ddd999a75"),
      "answers" :
      [
        {
          "userId" : "some GUIDs",
          "answer" : "apple"
        },
        {
          "userId" : "some GUID",
          "answer" : "apple"
        },
        {
          "userId" : "some GUID",
          "answer" : "banana"
        }
      ],
      "questionText" : "blah blah blah...",
      "questionType" : "multiple choice"
    }
]

Answer 1

有几种方法可以解决这个问题，具体取决于您需要处理的数据量。你可以使用 Aggregation Framework在 MongoDB 2.2+ 中，或者可能是 Map/Reduce .参见 Aggregation Commands Comparison有关功能和限制的摘要。

这是一个使用聚合框架的示例:

db.fruit.aggregate(
    // Limit matching documents (can take advantage of index)
    { $match: {
        "_id" : ObjectId("52c1d909fc7fc68ddd999a73")
    }},

    // Unpack the question & answer arrays
    { $unwind: "$questions" },
    { $unwind: "$questions.answers" },

    // Group by the answer values
    { $group: {
        _id: "$questions.answers.answer",
        count: { $sum: 1 }
    }}
)

对于您的示例文档，这将返回:

{
    "result" : [
        {
            "_id" : "banana",
            "count" : 1
        },
        {
            "_id" : "apple",
            "count" : 2
        }
    ],
    "ok" : 1
}